1. Field of the Invention
The invention relates to a program, a relay apparatus control method, a wireless communicating apparatus control method, and a system for connecting to an optimum access point which is fixedly placed in association with movement of a terminal and, more particularly, to a program, a relay apparatus control method, a wireless communicating apparatus control method, and a system for switching connection to an access point of optimum communicating conditions in the case where a terminal is positioned in a communication possible range where a plurality of access points are overlapped.
2. Description of the Related Arts
Hitherto, a wireless LAN has been known as a LAN which does not use a wired cable and the wireless LAN which conforms with IEEE802.11 has been widespread. The following three standards can be given as existing wireless LANs which conform with IEEE802.11: IEEE802.11b; IEEE802.11g; and IEEE802.11a. In IEEE802.11b, a radio wave of a band of 2.4 GHz is used, a spread spectrum communication system is used as a communication system, and a maximum transfer speed of 11 Mbps is realized. Likewise, in IEEE802.11g, a radio wave of a band of 2.4 GHz is used, an orthogonal frequency multiplex division system is used as a communication system, and a maximum transfer speed of 54 Mbps is realized. Further, in IEEE802.11a, a radio wave of a band of 5 GHz is used, the orthogonal frequency multiplex division system is used as a communication system, and a maximum transfer speed of 54 Mbps is realized.
In such a wireless LAN, a roaming technique is extremely important in the case where it is used in an infrastructure mode for connecting the wireless LAN to the wired LAN through an access point. The “roaming” is an operation for switching connection of a terminal in the case where the terminal moves out of a communication range of the access point to which the terminal is connected to at present into a communication range of another access point.
FIG. 1 shows an example of a fundamental construction of a conventional wireless LAN. Access points 100-1 and 100-2 of the wireless LAN have communication possible ranges 104-1 and 104-2 arranged so that placing positions can be covered without missing. Frequency channels for communicating have been preset into the access points 100-1 and 100-2. The available frequency channels have been predetermined every country and are as follows in Japan.
TABLE 1[  1]   IEEE802.11a5.150 · 5.250 MHz20 MHz  4  IEEE802.11b2.400 · 2.497 MHz 5 MHz  14  IEEE802.11c2.400 · 2.4835 MHz 5 MHz  13  (a) Standard(b) Frequency range(c) Channel allocation(d) 4 channels can be used at an interval of 20 MHz(e) 14 channels can be used at an interval of 5 MHz(f) 13 channels can be used at an interval of 5 MHz
In FIG. 1, terminals 102-1, 102-2, and 102-3 are connected to the access point 100-1, terminals 102-4 and 102-5 are connected to the access point 100-2, and the terminals 102-1 to 102-5 make communication in the areas of the communication possible ranges 104-1 and 104-2 where they are positioned at present, respectively. Different frequency channels are set into the access points 100-1 and 100-2 so that their mutual communication does not interfere with each other. In a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) as a communication system of the wireless LAN, when the communication is started, in order to confirm that another access point and the terminal do not make communication, a non-communication time of a predetermined time is detected and, thereafter, the communication is made. Therefore, if an access point or a terminal using the same channel exist in the neighborhood, they obstruct the mutual communication. To avoid such a situation, an operating method whereby different channels are set and the occurrence of the interference is prevented is used.
In FIG. 1, how the terminal is connected to the access point and the communication is started will now be described with respect to the terminal 102-2 as an example. A communication starting operation in the wireless LAN is executed by an access point searching operation and an associating operation.
(Access Point Searching Operation)
First, the terminal 102-2 transmits a probe request packet to search whether or not a connectable access point exists near when a power source is turned on. The probe request packet is transmitted to all channels which can be used by the terminal 102-2. This is because the terminal does not previously know to which channel the access point has been set. Although the access points 100-1 and 100-2 use the different channels, since the probe request packet from the terminal 102-2 is transmitted by using all of the channels, the access points receive the probe request packets of their own channels. After that, as a response to the terminal 102-2, each of the access points 100-1 and 100-2 returns a probe response packet showing that they can be connected to the terminal 102-2 as a response to the terminal 102-2. The terminal 102-2 which received the probe response packets from the access points 100-1 and 100-2 determines to which one of the access points 100-1 and 100-2 it is connected. An algorithm for such determination differs every apparatus.
(Associating Operation)
For example, when the access point 100-1 is determined as an access point to be connected to, the terminal 102-2 transmits an association request packet to obtain permission of the connection to the access point 100-1. The access point 100-1 which received the association request packet from the terminal 102-2 registers a terminal whose connection is permitted into an internal association table. The association table is a management table to store information of the terminal whose connection has been permitted by the access point 100-1. This table is held in the access point. After completion of the registration of the terminal 102-2 into the association table, the access point 100-1 notifies the terminal 102-2 of the connection permission by an association response packet. The associating operation is completed. After that, the terminal 102-2 communicates with the access point 100-1.
(Roaming Operation)
The operation when the terminal 102-2 executes the roaming on the wireless LAN with the construction of FIG. 1 will now be described. The operation when the terminal 102-2 moves out of the communication possible range 104-1 of the access point 100-1 which was registered first and the connection is restarted in the communication possible range 104-2 of the access point 100-2 is shown in FIG. 2.
First, it is assumed that a terminal 102-2″ moved like a terminal 102-2′ to the outside of the communication possible range 104-1 of the access point 100-1. Since the communication with the access point 100-1 connected to becomes impossible here, the terminal 102-2′ executes the operation to search for a new access point. Although this operation is the same as the access point searching operation described in FIG. 1, since the access point with which the communication can be made does not exist in this location, the transmission of the probe request packet is repeated until the probe response packet is returned. Subsequently, the terminal 102-2 continues to move and reaches the inside of the communication possible range 104-2 of the access point 100-2. The probe request packet from the terminal 102-2 is successively transmitted. Since the access point 100-2 exists at this point of time, the access point 100-2 which received the probe request packet returns the probe response packet. The terminal 102-2 determines the connection to the access point 100-2 and the access point searching operation is completed.
At this point of time, although the terminal 102-2 has determined the access point 100-2 to be connected to next, the association table of the access point 100-1 connected to before is not changed but keeps the contents that the terminal 102-2 has been registered. Therefore, the terminal 102-2 cannot make communication yet. Therefore, a reassociating operation is executed to update the association tables of the access points 100-1 and 100-2. In the reassociating operation, together with the making of the association table of the access point 100-2 to be newly connected to, a process for deleting the registration of the terminal which moved out of the association table of the access point 100-1 which has been connected to before is also executed.
The reassociating operation will now be described. The terminal 102-2 transmits a reassociation request packet to newly register the connection to the access point 100-2. Switching source access point information as information of the access point 100-1 to which the terminal had been connected is included in the reassociation request packet. The access point 100-2 which received the reassociation request packet from the terminal 102-2 additionally registers the terminal 102-2 into its own association table and instructs the access point 100-1, through a wired LAN 106, to which the terminal 102-2 had been connected to delete the terminal 102-2 from the association table. The access point 100-1 receives the deleting instruction from the association table of the terminal 102-2 from the access point 100-2 and deletes the terminal 102-2 from its own association table. By transmitting a reassociation response packet, the access point 100-2 notifies the terminal 102-2 that the reassociating operation has been completed. The terminal 102-2 receives the reassociation response packet and completes the reassociating operation. The roaming operation is completed in this manner. After that, the terminal 102-2 communicates with the access point 100-2.
However, according to the conventional roaming in the wireless LAN, there are the following problems.
FIG. 3 shows a case where a problem occurs in the conventional roaming operation. In the construction of the wireless LAN, an overlap of the communication possible ranges 104-1 and 104-2 of the access points 100-1 and 100-2 is large. Such an environment can occur in the case where a transmission electric power and receiving sensitivity of each of the access points 100-1 and 100-2 are large and their communication ranges are large, in the case where the positions of the access points 100-1 and 100-2 are close, or the like. Association situations of the access points 100-1 and 100-2 and the association tables are similar to those in FIG. 1. The terminal 102-2 is initially located in the position of the terminal 102-2′. It is assumed that the terminal 102-2 moved near to the access point 100-2 in such a state. However, since the terminal 102-2 still remains in the communication possible range 104-1 of the access point 100-1, it does not execute the roaming operation but continues to communicate with the access point 100-1.
Considering the positional relation of the terminal 102-2, in spite of the fact that the terminal 102-2 is near the access point 100-2, the communication is made with the access point 100-1. The problem in this case is that communication quality of the wireless LAN is largely influenced by the distance. When the distance is short, the communication quality is hardly influenced by other radio waves or obstacles and performance is high. However, when the distance is long, the performance contrarily deteriorates. Therefore, when the terminal 102-2 moves near to another access point 100-2, even if it is within the communication possible range 104-1 of the access point 100-1 connected to at present, communicating with the access point 100-2 which is near would improve the performance. However, such a communication construction is impossible in the conventional roaming operation.
As mentioned above, in the conventional roaming operation, even when the access point of the high communication quality exists near, switching of the access point is impossible and the terminal can communicate only with the access point of the inferior communication quality.
The roaming system of the wireless LAN disclosed in JP-A-09-083545 has been made on the assumption that all channels (communicating frequencies) of the adjacent access points are the same. The terminal in JP-A-09-083545 detects the access points by receiving a beacon. The beacon is a packet which is periodically transmitted from the access point to the terminal and transmitted by the channel which has been preset in the access point.
The terminal receives the beacons from a plurality of access points and measures intensity of the radio wave during the communication. However, this method can be realized because all of the access points use the same frequency channel. If a different frequency channel is set every access point, the terminal cannot receive the beacons of the adjacent access points. In the wireless LAN in which the different frequency channel is set every access point as shown in FIGS. 1 to 3, the roaming operation according to JP-A-09-083545 cannot be executed.